This invention relates generally to electrically-actuated braking systems for towed vehicles and the like, in particular the control means for such systems. More particularly still, the invention pertains to inertially-responsive control means for such braking systems, and in particular to inertial sensors, or accellerometers, for use therein.
Electrically-actuated braking systems for towed vehicles have, of course, been known and used for some time, as shown for example by earlier U.S. Pat. Nos. 3,738,710, 3,953,084, 3,967,863, 3,981,544, 3,909,075, and 4,030,756 (certain of which are related to one another) which show various attributes of such systems and additionally discuss the background and historical developments in this field. While early such systems were manually-controlled, subsequent developments utilized various means for achieving automatic actuation, and such automatic systems have come to rely upon inertial sensors as the most predominant such control means since they enable the towed vehicle ("trailer") brakes to be applied automatically as and when braking is desired. Of course, many particular developments have occurred over the years for enhancing and improving such inertial systems, as shown by the earlier patents noted above, together with those cited therein, etc.
What is believed to be the most successful and most frequently-utilized state of the art inertial sensing system is one analogous to that shown in the aforementioned U.S. Pat. Nos. 3,967,863 and 3,981,544, which are assigned to the assignee of the present invention. This system uses a pendulum-type inertial sensor, or accellerometer, which responds to braking of the tow vehicle by immediately causing a controlled application of the brakes on the towed vehicle. As disclosed in these patents, the pendulum utilized in this system comprises a "sandwich" of laminated members, e.g., a pair of outer layers of thin sheet metal, in particular beryllium copper, and a center layer of yieldable, elastomeric material, to which the outer metal layers are adhered. This construction is utilized to provide self-damping pendulum deflection, since the pendulum is mounted from its top and flexes laterally from the bottom when subjected to orthogonally-oriented acceleration forces, the pendulum being weighted at the bottom to augment such flexure.
This lateral deflection of the pendulum requires that its two sheet metal layers move longitudinally with respect to one another, and this causes the elastomeric layer adhered therebetween to undergo elastic deformation since it is subjected to shear forces as a result of its side extremities moving in opposite longitudinal directions. This elastic deformation of the resilient inner layer of the pendulum is intended to damp pendulum deflection in a desirable manner, and to augment return of the pendulum to its normal position as inertial forces diminish, thereby contributing to smooth and controlled braking response as well as helping to remove the braking effects as quickly as possible when they are no longer needed.
In accordance with the present invention, it is perceived that the desirable effects just noted were to a considerable degree sacrificed by the particularities of the pendulum configuration and mounting techniques used heretofore, which in fact have been the source of undesirable but largely unappreciated problems occurring in actual usage. Thus, for example, prior art implementations of the multiple-layer leaf-type pendulum have failed to provide the desirable benefits of its inherent self-damping capability, and have in fact introduced eccentricities and irregularities in the inertial displacement of the pendulum, resulting in inappropriate and undesirable braking effects exerted upon the towed vehicle, erratic in nature and disproportionate to the actual inertial effect causing the response. Indeed, at times the resulting braking response would be too great, or too small, while at other times the result could be chatter (i.e., rapid on and off or increase and decrease of braking effect), caused by resonance or other vibratory mechanical oscillation in the sensing pendulum.
To a considerable extent, the adverse effects just noted have resulted from the manner in which the laminate-form leaf-type pendulum was mounted for inertia-induced flexure. That is, the multi-layer "sandwich" structure of the pendulum was typically mounted by use of a clamping means by which the top of the pendulum was clamped to a fixed, rigid mount or support structure. While some such suspensions also used a right-angled offset portion of one or both of the sheet metal layers of which the pendulum was formed to provide a means for suspending the pendulum from its top, the pendulum has heretofore also been rigidly secured by a clamp, which in effect squeezed the various layers of the pendulum against one another at the uppermost end, holding them tightly against a support disposed parallel to the axis of the pendulum. While this captured the upper end extremity of the pendulum and supported it in a dependable manner, it also caused serious but unappreciated problems. That is, as noted above, the lower extremity of the pendulum carried a weight, whose purpose is to accentuate the inertial effect on the pendulum caused by braking of the towing vehicle, and the customary manner of securing this weight to the lower extremity of the pendulum securely captures the entire lower extremity of the latter, i.e., both metal layers as well as the elastomeric internal layer. Accordingly, both the upper and lower extremities of the pendulum were rigidly captured relative to one another, with the result that the different layers were unable to move longitudinally relative to one another as the pendulum underwent lateral flexure, except to the very limited extent that such motion was made possible through tensional lengthening of one metal layer accompanied by compressive shortening of the other, and the extent to which the metal layers could separate by bowing and moving away from one another, stretching the elastomeric intermediate layer laterally through tension. These unintended and unappreciated effects changed the spring action of the pendulum dramatically, making it uncoordinated and irregular, while at the same time substantially nullifying the otherwise-obtainable smooth, coordinated, elastically-damped flexure of the pendulum, causing it to act erratically, in extreme cases actually causing it to "oil-can," i.e., erratically snap over-center, with resulting erratic braking action.
A further anomaly, probably based upon misconception, exhibited by prior pendulum-type inertial sensors of the type described is the use of comparatively long pendulum blades, by which comparatively large flexural excursion was obtained. This was thought necessary in order to provide desired resolution in the system, since the pendulum excursion is used to produce a graduated, proportional response for use in controlling the application of the towed vehicle brakes. That is, the pendulum motion is used to produce a corresponding analog signal in accordance with which braking is effected. For example, the pendulum motion may be used to either occlude or open a path for a light beam extending between a source and a photo detector, such that higher inertial levels causing greater pendulum deflection correspondingly increase the amount of light sensed by the photodetector. However, while it may be thought to be easier, as an abstract matter, to obtain higher degrees of control resolution by use of larger pendulum deflection, this is not the case where the longer pendulum contributes directly to instability of the aforementioned nature, creating a situation in which deflection increments ar not uniform in relation to the corresponding inertial effects, and are not consistent.